1. Field of the Invention
This invention relates generally to feedback control of oximeter diode drive currents and, more particularly, to eliminating oscillations due to diode cable reactances without degrading current control.
2. Related Work
A pulse oximeter is a type of blood gas monitor which non-invasively measures an amount of saturation of oxygen in the blood. The saturation of oxygenated blood may be determined from the differential absorptions for two plethysmographic waveforms measured at separate wavelengths. The two waveforms are typically produced by driving a visible red light-emitting diode (LED) and an infra-red LED to produce two lights that pass through a patient""s tissue, and then detecting the light on the same or an opposite side of the tissue using one or more photodetectors. The light-emitting LEDs are placed in a probe that is attached to the patient""s body in a preferred location for the particular application. Although most conventional oximeters use the red and infra-red LEDs, other devices such as surface emitting laser devices having different wavelengths may also be used, and the number of LEDs can vary according to the specific measurement application. For example, it is known to set a number of laser diodes to be equal to or than the number of blood analytes that are to be measured by the instrument. In the specific field of photoplethysmography, the light beams that are generated by the LEDs must be of sufficient intensity to illuminate the perfused tissue and also be of constant wavelength, since the light absorption of the monitored analyte varies as a function of wavelength.
The probe may be a sleeve or clamp that fits around a patient""s finger or earlobe. The LEDs are disposed in the probe, for example, to be positioned on one side of the patient""s finger. The probe is electrically connected to an LED drive circuit and to detection measurement and analysis apparatus via a cable of a given length.
The probe also has one or more photodetectors that detect the light, for example, received on an opposite side of the patient""s appendage from the LEDs. The received light of different wavelengths is converted into electrical signals by the photodetector. The signals are then electronically processed and analyzed to isolate signals representing a measurement of oxygen saturation of arterial blood.
Ideally, the current source for an oximeter LED simply drives current through the probe cable and into the LED without the current being affected by its transmission through the LED driver circuit and probe cable. However, the characteristics of the cable and LED affect the stability of the oximeter LED drive circuit. Large changes in voltage can occur across the probe cable when the LED turns on or off, where this LED switching can cause the oximeter LED drive circuit to experience oscillation as the LED current control circuit tries to keep up with the changing cable voltage. Therefore, when the diode is switched on and off, an oscillation current (ringing noise) may be superimposed on current flowing through the diode. This oscillation degrades current control, and affects accuracy and sensitivity of oximetry measurements. It is understood that the term xe2x80x9ccurrent sourcexe2x80x9d used herein refers to either or both of a sourcing or a current sinking configuration.
This oscillation problem exists because the op-amp used to provide the gain in the feedback circuit is slow, the transistor used to control the current is slow, and the current feedback signal is slower still because of parasitic capacitances. Oscillations result when the combined sluggish response of the op-amp, transistor, and feedback is so slow that the feedback response contributes to amplifying the next oscillation instead of contributing to damping. This sluggishness of response is commonly called xe2x80x9cphase lag,xe2x80x9d the condition of oscillating occurring when the phase lag is greater than 180 degrees and when the circuit is still amplifying (gain greater than 1). When the lag is greater than 180 degrees, the feedback starts to return back to a same part of a cycle, causing oscillation. When the lag is 360 degrees, the feedback comes completely back to the same part of the oscillatory cycle. In this manner, the switching of diode voltages can cause LED cable oscillations.
Conventional methods for addressing oximeter diode driver performance have included, for example, setting respective references for the drive voltage at a first voltage level and a second voltage level, and then supplying current to emitter circuits only when the drive voltage is at a first voltage level. Such a method sets the selected voltage reference to a value that determines the magnitude of the desired drive current. However, such conventional systems do not correct a phase lag of a diode drive circuit, but merely compensate for a slow voltage rise time by adjusting a corresponding timing for supplying a drive current.
As noted above, the switching of voltages in an oximeter can cause LED cable oscillations. This problem is compounded by the need for lowering a power consumption and cost of an oximeter. The conventional diode drive circuit discussed above is inefficient and expensive because it requires multiple reference amplifiers and prolonged stabilization periods between switching. Such a drive circuit necessitates a changing of timings for driving the diode
Another conventional oximeter diode drive circuit attempts to compensate for lag by lowering the slew rate of the output voltage to the diode, and by using additional load capacitance. However, the voltage across the capacitor will bleed down, causing a power loss, and oximeter performance is reduced significantly by a decoupling of the drive current to the diode. In addition, adding the load capacitor does not reduce a generation of front end noise, does not stabilize a control of the current driving, and does not eliminate any noise from a drive circuit itself.
An additional conventional method uses a low gain in a feedback loop of a driver circuit. However, this greatly reduces a performance of a driver circuit and also does not actually reduce noise, but merely reduces a dynamic range for diode drive current control and reduces an amplitude of a resultant ringing.
What is needed is a method and apparatus for stabilizing the driving of a current through a diode of an oximeter, where a low cost and a low power consumption are achieved along with tight control of the current through the diode. In addition, a clean diode current source with expanded dynamic range is needed for improving resolution and signal-to-noise ratio in oximetry systems. The effects of front-end, diode current noise are compounded by a use, for example, of laser diodes, which have very non-linear characteristics compared with conventional LEDs. This non-linearity of a laser diode means that a high-bandwidth laser diode cannot withstand an overcurrent state due to oscillations, even for a short period of time. As a result, conventional diode drive circuits may cause a complete breakdown of a laser diode. An example of a use of laser diodes in pulse oximetry is disclosed in U.S. Pat. No. 6,253,097 to Aronow, et al., which is incorporated by reference in its entirety.
It is an object of the present invention to prevent oscillations caused by sluggish performance of a diode driver circuit.
It is another object of the present invention to correct phase lag of a diode driver circuit without requiring that a timing of switching of the diode be changed.
It is a further object of the present invention to improve over conventional diode driver circuits by allowing for faster switching and more accurate control of multiple diodes.
It is an additional object of the present invention to stabilize a feedback type current source with a low-cost ferrite bead.
It is a still further object of the present invention to stabilize a current source that includes a current-sensing feedback arrangement.
It is yet another object of the present invention to identify a noise source in a diode current sourcing circuit and then reduce that noise, with or without affecting a current passing through the diode.
It is another object of the present invention to maintain a tight control of diode current in a diode driver circuit of an oximeter.
The present invention provides an apparatus including an amplifier having feedback, a resistor that senses a current being input to the amplifier, and an inductor that adds phase lead to the current.
A method for driving a current through a diode of an oximeter according to the present invention includes sensing a current of a signal being input to a feedback type amplifier, adding phase lead to the signal being input to the feedback type amplifier, and driving a diode with at least part of the signal having added phase lead.
A method for driving a current through a diode of an oximeter according to the present invention includes driving a current through the diode by connecting the current to the diode, controlling the connecting using a feedback type amplifier, the feedback type amplifier having a current source that uses a current-limiting resistor, and adding phase lead to a current passing through the current-limiting resistor.
An oximeter diode-driving device according to the present invention includes a diode, a transistor operative to controllably connect current to the diode, a current sensing resistor that senses at least part of the current, a feedback amplifier operative to control the connecting of current by the transistor according to the current sensed by the current sensing resistor, and an inductor in series with the current sensing resistor.
As a result of implementing the present invention, oscillations due to diode cable probe reactances are reduced or eliminated without degrading current control. In addition, dynamic range is increased, and power consumption and cost are reduced.
These and other objects, features, and advantages of the present invention will become more apparent when considered in connection with a detailed description of preferred embodiments, which are illustrated in the accompanying drawing figures.
This summary does not limit the invention, which is instead defined by the appended claims.